Fixed ISO C99 compatibility

SDL now builds with gcc 7.2 with the following command line options:
-Wall -pedantic-errors -Wno-deprecated-declarations -Wno-overlength-strings --std=c99

src/thread/windows/SDL_systhread.c

@@ -178,7 +178,7 @@ SDL_SYS_SetupThread(const char *name)
         if (!kernel32) {
             kernel32 = LoadLibraryW(L"kernel32.dll");
             if (kernel32) {
-                pSetThreadDescription = (pfnSetThreadDescription) GetProcAddress(kernel32, "SetThreadDescription");
+                *(void**)&pSetThreadDescription = GetProcAddress(kernel32, "SetThreadDescription");
             }
         }